Amplifying circuits



P 1933. w, VAN B. ROBERTS I 1,925,569

AMPLIFYING CIRCUITS Filed Oct. 26. 1928 k iim mmmh INVENTOR AN B. ROBERTS A ORNEY Patented Sept. 5, 1933 AMPLIFYING CIRCUITS Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 26, 1928. SerialNo. 315,142

16 Claims.

This invention relates to circuits for amplifying feeble pulsating electric currents. More particularly it relates to an amplifying circuit, or

stage, including a space discharge device, in which the circuit connections are such that a high amcircuit superposed upon the steady plate cur rent. .If the plate circuit contains no impedance the magnitude of the alternating current is the product of the voltage applied to the grid and r the mutual conductance of the tube. When, however, an impedance is inserted in the external plate circuit, the current flowing through this impedance produces an alternating variation in the potential of the plate. This alternating plate potential acts to partially oppose the effect of the input voltage impressed upon the grid and thus prevents the plate current from attaining the value it would have if there were no impedance in the plate circuit;

It is the object of my invention to provide'an amplifying circuit including a thermionic valve in which the space current is not substantially affected by plate voltage variations of the type which is 180 out of phase with the aforesaid 40 anode potential variations and which has a value equal to 1 i. e., the product of the reciprocal of the amplification constant of the valve and the anode potential variation (ea).

As a

the plate current occurring as a result of the variation of the anode potential, is offset by the oppositely phased but equally effective voltage impressed upon the grid so thatthe space current result of this arrangement, any tendency to alter.

isnot affected. It is understood, however, that.

input voltages applied to the grid will continue to control the plate current exactly as if no load were present in the plate circuit.

My invention is also applicable to valves of the shielded grid type, particularly when their construction is such that the shielding action is not perfect. When the shielding action is perfect, variations in the anode potential do not affect the space current and as a result the shielded grid valve gives a very high amplification. But when the shielding action is not perfect the variations in the anode potential affect the space current of the four element valve in substantially the same manner as in the three element valve although not to so great an extent.

A further object of my invention is to provide an amplifying circuit adapted to use a shielded grid, valve not having perfect shielding action, with means for preventing variations of anode potential from affecting the flow of space current.

This object is attained by applying to the screen grid a potential which is 180 out of phase with the aforesaid anode potential variations and which has the proper value with respect to the value of the anodepotential variations, to maintain the space current substantially unaffected byjsaid anode potential variations.

A further application of my invention is to maintain a constant current in'a circuit that is subject to current and voltage pulsations. A

good example of such a circuit is a power supply circuit for rectifying alternating current, and smoothing out the rectified current to provide power for operating a radio communication apparatus. It is a still further object of my invention to provide means in a circuit subject to current and voltage pulsations, for maintaining a constant current in the circuit.

In the case of a power supply circuit, the ob-- ject of myinvention is attained by including a space discharge device having a control grid in the power supply circuit so that the circuit is completed by the space current of the device. Means are then provided for impressing upon the grid of the device a potential which is 180 out of phase withthe voltage pulsations that tend to vary the space current through the device. The means are such that the reverse potential 105 has a value equal to result the space current is changed in such a 11.0

the-methods well known in the art.

manner that the current through the device remains constant regardless of the potential variations tending to pass more current thru the circuit.

Other objects of my invention will become apparent upon reading the following specification in connection with the accompanying drawing showing various modifications of my invention in which: 7

Figure 1 is a diagrammatic illustration of the application of the broad principle of my invention to an amplifying circuit employing a three element valve; V

Figure 2 illustrates the broad application of my invention to an amplifying circuit employing a four element or shielded grid valve; 7

Figure 3 illustrates in detail a receiving circuit including two amplifying stages according to my invention;

Figure 4 illustrates the application of my invention to the frequency changing circuit of a superheterodyne receiver;

Figure 5 illustrates in detail an amplifying circuit employinga four element or shielded grid valve, to which my invention has been applied;

Figure 6 illustrates an amplifying circuit including a three element valve, that is modified in another way in accordance with my invention; and, v

Figure '7 illustrates an amplifying circuit including a three element valve, that is modified in still another way in accordance with my inven tion.

Referring more particularly to the figures in the modification illustrated by Figure 1 a space discharge device of the three element type is provided with an input circuit including an induc- 'tance 2, and an output circuit including an in- "ductance 3.

An inductance 4-, which is formed in this case by continuing the winding which forms inductance 3, is also included in the input circuit. A tuning condenser 5 is connected in parallel with the series arrangement of inductances 3 and 4 but the circuit would operate equally well if the condenser 5 was only connected in parallel with the inductance 3. The currents to be amplified are supplied to the amplifying circuit by a circuit 6 coupled to inductance 2, and a circuit '7 iscoupled to inductance 3 in the output circuit. In case parasitic oscillations are met with in this or any of the following circuits, they may beovercome by any of For instance, a trap circuit (which is merely an oscillation circuit tuned to the parasitic frequency) may be coupled to the coil carrying the parasitic current.

By making the inductances 3 and i in a single winding, one end of which is connected to the anode and the other end of which is connected through inductance 2 to the grid, every change in anode potential produced by the signals causes a potential to be impressed upon the grid which is 189 out of phase withthe change in anode potential. Thus, when the anode potential decreases as aresult of an incr asing current through the inductance 3, the grid is made more positive so that the space current does not Vary as a result of the decreasedanode potential.

In order to maintain the space current constant it is necessary for the potential impressed upon the-grid to have a magnitude or value equal to the product of the anode potential variation multiplied by the reciprocal of the amplification constant of the valve. According to one form of put circuits in the usual manner. impressed upon the input circuit through a cirmy invention, this is accomplished by making the ratio of the voltage drop across inductance l to the voltage drop across inductance 3 equal to It is not necessary that the inductance 4 be made part of the same winding as inductance 3, as shown in Figure 1, out instead the inductance fl can be a separate winding inserted in the input ductances 4 and 3 is less than if the magnitude of the potential impressed upon the grid will be less than the amount necessary to maintain the space current unaiiected by plate potential and consequentlythere is a decrease in the space current with a corresponding decrease in amplification from what could be obtained by having the ratio exactly If the ratio is greater than 1 E the space current instead of being maintained unaffected will be increased and the circuit will tend to set up oscillations. In practice, due to the difliculty in obtaining the exact ratio it is usually made slightly less than 1 to obtain an amplification slightly less than the maximum that could be obtained out which at the same time is much higher than the amplification constant of the ordinary amplifying circuit. As long as the ratio is or less, the circuit can not oscnlate regardless of the resistance of the circuit.

My invention is also applicable to amplifying circuits employing a shielded grid valve, as shown by Figure 2. In this modification a four element valve 3 which is assumed to have imperfect. shielding, is provided with'input and outcuit 6 coupled to inductance 2 or in any other well known manner. The output from the amplifying circuit is obtained in any well known manner; Means are also provided in this circuit for impressing. upon the shielding grid a potential opposite in phase to the anode potential fluctuations and having a magnitude such that the space current is maintained unaffected by variations in the anode potential, such as those produced by signals. Thisreverse potential may be obtained as in Figure 1, by an inductance tightly coupled to the output coupling inductance or it maybe obtained by means of a high impedance winding 9 connected across the output circuit and a winding 10 coupled thereto as shown. As the winding 9 is not used to transfer energy from the output circuit to a load, it is givensuch a high impedance that there is practically no current fiow but only a potential drop between the ends of the winding.

In the modifications described the inter-electrode capacities have been neglected and the effect of tuning the input circuit has not been considered When the input circuit is tuned it is necessary to neutralize all feed-back from the Signals are output circuit to the input circuit, that is, means must be provided for preventing the oscillations in the output circuit from setting up oscillations of a regenerative nature in the input circuit. Figure 1, when so modified, becomes the radio frequency amplifying circuit illustrated by Figure 3 or the superheterodyne circuit illustrated by Figure 4, and Figure 2 becomes the amplifying circuit illustrated by Figure 5. V

Figure 3 illustrates a complete receiving circuit including two stages of radio frequency amplification modified according to my invention. In each of the amplifying stages in this-circuit, the inductance 2 issplit to provide an intermediate tap 11, to which the cathode is connected ductance 3 and arranged to have the same ratio with inductance 3, as in the modification illustrated by Figure 1.. The tuned output circuit of the amplifyingstage can be inductively coupled with the tuned input circuit of the succeed ing stage as shown by the coupling between the two amplifying stages, ora single tuned circuit can be used as shown by the coupling between V the detector and the preceding stage.

Figure 4 illustrates the application of my invention to a heterodyne circuit in which the input circuit and the output circuit are tuned to different frequencies. The inductance 2 which is tuned by the variable condenser 13, is coupled to an antenna circuit 14 and a local oscillator 15. The output circuit contains the usual inductance 3 which is connect-ed in parallel with the fixed condenser 16. As the input circuit and output circuit are tuned to different frequencies it is not necessary to provide means for preventing oscil-' lations in the output circuit from setting up oscillations in the input circuit. A condenser 17 is provided as a radio frequency by-pass. As in the previous cases a reverse potential of the proper magnitude is impressed upon the grid by means of the inductance 4.

In the modification illustrated by Figure 5 a four element or shielded grid tube which is considered to have an imperfect shielding action, is provided with tuned input and output circuits. The input inductance 2 which is tuned by means of the variable condenser 13, has one end connected to the grid, an intermediate point 11 connected to the cathode and the other end connected to the anode through a condenser 12, as in Figure 3. This arrangement. is provided to balance out any capacitive feed-back from the output circuit leads and tuning elements to those in the input circuit. The circuit will also balance out any inter-electrode capacitive feedback which might exist due to the imperfect shielding action of the valve. The output circuit contains the usual inductance 3 and tuning condenser 5. An inductance 4' which corresponds to the inductance 4 in Figure 3, is connected between the cathode and the shielding grid as shown. The inductance is positioned so as to be tightly coupled to inductance 3 in such a manner that a reverse potential of the proper magnitude to maintain a constant space current, will be impressed upon the shielding grid.

In the modifications illustrated by Figures 6 and 7 a somewhat different arrangement is provided for obtaining the same results. In Figure 6 the input circuit inductance, corresponding to inductance 2 in Figure 3, is analyzed into two inductances l8 and 19 which have no mutual inductance with other coils and a Winding 20 having an intermediate point connected to the cathode. The two halves of inductance 20 are coupled 100%. The input circuit is tuned by the variable condenser 13 connected in parallel with the inductances 18, 19 and 20. A variable inductance 21 is provided in the input circuit between the cathode and the connection to inductance 20. The inductance 18' is connected to the grid and the inductance 19 is connected to the anode through the capacitance 22. The inter-electrode capacity is indicated at 23.

The theory of this circuit is as follows:

Assume an anode potential having a value of Ga. This potential requires the existence of a current flowing from the anode to the cathode through two paths, one of the paths comprising the inter-e1ectrode capacity 23, inductance 18, part of inductance 20 and inductance 21, and the other path comprising the capacitance 22, inductance 19 part of inductance 20 and inductance 21.

where V is the inductance of winding 21, L is the inductance of winding 18 or winding 19, and C is the capacity of condenser 22 or the inter electrode capacity 23. The grid potential resulting from this current is Now c will'be that is the grid will have the proper magnitude of reverse potential, if

1 1 m=(u Hence lay-adjusting V or C or both, that is, by

adjusting inductance or capacitances 22 and 23, or both, the proper magnitude of reverse potential can be impressed upon the grid. However, this adjustment only holds good for one frequency and must be varied as the frequency is varied. 1

Figure '7 illustrates a circuit which is a slight modification of Figure 6. .In this modification the input circuit is tuned by means of Variometers 24 and 25 of equal inductance which are substituted for the inductance 18, 19 and 20 in pacitances' 22 and 27 can be adjusted so that a potential of the proper magnitude is impressed upon the grid at all frequencies without any other adjustments than the tuning of the input circuit.

In all of these modifications, as a result of the This current has a value equal to f lid increased amplification, it is not only necessary that the adjustment for neutralizing the feedbacks be carefully made, but also symmetry in the circuits is advisable to obtain the best results. For example, in the modification illustrated by Figure 3 it might be advisable to add a small condenser between the lower end of the input circuit inductance 2 and the cathode in order to balance the grid-cathode capacity.

Various other modifications of the circuits disclosed and various other applications of the broad principle of my invention have been contemplated by me and I do not intend to be limited by the modifications disclosed but only by the scope of the appended claims.

What I claim is:

1. The method of maintaining the space current of a space discharge device substantially unaffected by the alternating potential impressed upon the plate electrode of said device by the variations amp. iied therein which consists in varying the apparent impedance of the space discharge path of said device proportionally to ti. e variations of the alternating potential.

2. The step in a method of amplifying electric currents in an amplifying circuit including a multi-elernent valve which comprises maintaining the space current of the valve substantially independent of the impedance of the load circuit when signals are impressed on the input of the valve by superposing on the input circuit an alternating voltage of phase opposite to the voltage developed across the load and of magnitude to substantially neutralize the effects of said voltage developed across the load upon the space current.

3. The step in a method of amplifying electric currents in a circuit including a valve having a cathode, a grid, and an anode and inwhich there is a varying difierence of potential between the anode and the cathode as the result of the flow of currents in the output circuit, which comprises impressing upon the grid a potential equal to the sum of the input potential and a potential which i is equal to the product of the reciprocal of the valve amplification constant multiplied by the anode potential fluctuations, and opposite in phase thereto.

l. The method of increasing the amplification of an amplifying valve having a cathode, a grid, and an anode and in which there is a varying dif ference of potential. between the anode and the cathode as the result of the flow of currents in the output circuit of said valve, which consists inimpressing upon said grid 2. potetnial which varies in proportion to the anode potential variations but in opposite sense thereto and which has a magnitude such that the space current of said valve'is substantially unaffected by said anode potential variations.

5. A. circuit comprising a space discharge device having input and output circuits, an anode in said device upon which is impressed a potential that varies according to the flow of current through said output circuit, a control electrode in said device, means for varying the potential of said control element according to the current to be amplified, and means comprising a connection between. the input and output circuits for maintential that varies according to theflow of current in said output circuit, means for supplying thecurrents to be amplified to said device, and means comprising a'current path between input and output circuit for preventing the variations in the potential impressed upon said anode from substantially changing the space current of said device.

7. An amplifying circuit comprising an amplitying valve, a plurality of elements including an anode, a cathode and a control electrode in said valve, circuits connected between said cathode and said control electrode and between said cathode and said anode whereby a varying difference of potential is obtained between said cathode and said anode, and means for impressing upon one of said valve elements a potential 180 out of phase with said varying difference of potential and having a magnitude approximately equal to the product of said varying difference of potential multiplied by the reciprocal of the valve amplification constant.

8. An amplifying circuit comp-rising a valve including an anode, a cathode and a control electrode, an inductance having one endconnected to said anode, a connection between an intermediate point on said inductance and said cathode, a second inductance, a direct current connection between the other end of the first mentioned inductance and said control electrode, connection including second inductance and a circuit coupled to said second inductance for supplying the currents to be amplified to said amplifying circuit.

9. An amplifying circuit comprising a space discharge device containing an anode, a cathode and a control electrode, an inductance having one end connected to said control electrode and its other end connected througha condenser to said anode, and a second inductance having one end connected to said anode, an intermediate point connected to said cathode and its other end connected to an intermediate point on the first mentioned inductance.

10. An amplifying circuit comprising a space discharge device including an anode, a cathode and a control electrode, an inductance having one end connected to said control electrode, a connection from the other end of said inductance to said anode, a second inductance having one end connected to said anode, a connection from an intermediate point of said second inductance an output circuit for said valve, an anode in said valve upon which is impressed a potential that varies in accordance with the flow of curvrent through said output circuit, a control electrode in said valve, means for impressing potentials upon said control electrode which bear a definite relation to the potential variations of said anode, and means for balancing out the coupling between the output and input circuit due to the capacitive path between said anode and said control. electrode.

12. A method of maintaining the space cur- .rent of a space discharge device substantially unaffected by the alternating component of diference of potential produced between the plate and cathode thereof by the flow of the output current through the external plate circuit impedance, which consists in superposing on the grid potential an alternating potential proportional to the alternating component of the potential difference between the plate and cathode but opposite thereto in phase.

13. In an electrical relay, a space discharge device having an input circuit and an output circuit, means for maintaining the space, current of said device substantially independent of the value of the impedance in the output circuit when signals are impressed on the input circuit, comprising means for feeding back energy from the output circuit to the input circuit for superposing on the input circuit an alternating voltage having a phase opposite to the voltage developed across the output circuit impedance and of a magnitude sufficient to neutralize he effects of the voltage developed across the output impedance upon the space current of the device.

14. In an electrical relay having an input circuit and an output circuit, a space discharge path device including anode, cathode and at least one control electrode, a source of anode potential connected between said anode and cathode, means for impressing signal energy upon the input circuit of the relay whereby currents are caused to flow in the output circuit bearing a certain predetermined relation to the impressed signal energy, said output circuit presenting certain inherent impeding effects to the flow of cur-- rent therein causing a voltage to be developed within said circuit which varies the normal anode potential in a sense tending to oppose current flow in said output circuit, means for impressing electric potential effects, in the discharge path of said device, derived from the anode potential variations in said output circuit, of such phase and magnitude relative to the developed impeding voltage as to neutralize said impeding effects, whereby the output circuit current pro-- duced by signals impressed on the input circuit is maintained substantially independent of the value of the output circuit impedance.

15. In an electrical relay having an input circuit and an output circuit, a space discharge path device including anode, cathode and at least one control electrode, means for impressing signal energy upon the input circuit of the relay whereby currents are caused to flow in the output circuit bearing a certain predetermined relation to the impressed signal energy, said output circuit presenting certain inherent impeding cheats to the flow of current therein causing ing a voltage to be developed within said circuit which varies the normal anode potential in a sense tending to oppose current flow in said output circuit, means for impressing upon a control electrode of said device a voltage derived from the anode potential variations in said output circuit of such phase and magnitude relative to the developed impeding voltage to neutralize said impeding effects whereby the output circuit current produced by signals impressed on the input circuit if maintained substantially independent of the value of the output circuit impedance.

16. In an electrical relay having an input circuit and an output circuit, a space discharge path device including an anode, a cathode, a control electrode and a screening electrode, a source of anode potential connected between said anode and cathode, means for impressing signal energy upon the input circuit of the relay whereby currents are caused to flow in the output circuit which bear a certain predetermined relation to the impressed signal energy, said output circuit presenting certain inherent impeding effects to the flow of current therein causing a voltage to be developed within the circuit which varies the normal anode potential in a sense tending to oppose current flow in said output circuit, means for impressing upon said screening electrode a potential derived from the anode potential variations in said output circuit of such phase and magnitude relative to the developed impeding voltage as to neutralize said impeding efifects, whereby the output circuit current produced by signals impressed on the input circuit is maintained substantially independent of the magnitude of said output circuit impeding effects.

WALTER VAN B. ROBERTS. 

